Day length predicts investment in human immune function: Shorter days yield greater investment

A multi-site study of the relationship between photoperiod and ovulation rate using Natural Cycles data

“…Specifically, we are unable to determine whether the observed relationship between day length and ovulation rate has emerged in response to greater investment in reproductive function during the summer months or decreased investment in reproductive function during the winter months. Given past research finds increased investment in immunological activity and concomitant reductions in total levels of the sex hormone testosterone in men during shorter days 12 , the effects observed in the current results may be (a) driven by adaptations designed to promote reproduction in the spring and summer months or (b) a byproduct of increased investment in bodily maintenance and repair during the fall and winter months. Future work would benefit from identifying the mechanisms that underlie these seasonal patterns to better understand what, if any, adaptive function they might serve.…”

“…While day length is a powerful predictor of upcoming seasonal changes for a wide variety of species 62 – 64 , including humans (e.g. 12 ,), it is possible that it is not the preeminent seasonally-variable cue that modulates women’s ovarian function (e.g. 34 ,).…”

“…Such protections are a relatively new fixture in human evolutionary history, however. One of the legacies of these exposures is that many human biological systems—such as those that govern energy expenditure 35 , hormone release 36 – 39 , and immune function 12 —vary seasonally, similar to what is observed in seasonal breeders. For example, research finds powerful effects of photoperiod on multiple facets of immunological function, such as peripheral blood mononuclear cell proliferation 12 which each peak in the winter months, when exposures to harsh environmental conditions would have historically been high 38 , 40 , 41 .…”

Human immune and metabolic biomarker levels, and stress-biomarker associations, differ by season: Implications for biomedical health research

“…Consistent with the possibility that human biomarkers vary across the year, observational research examining seasonal shifts in human immunity found that shorter day lengths were associated with higher circulating inflammatory markers, and greater peripheral blood mononuclear cell proliferation and cytotoxicity ( Gassen et al, 2019 ). Corroborating results were found using data from both children and adults who participated in the National Health and Nutrition Examination Survey ( Liu and Taioli, 2015 ), as well as UK Biobank participants ( Wyse et al, 2021 ).…”

Photoperiod Conditions Modulate Serum Oxylipins Levels in Healthy and Obese Rats: Impact of Proanthocyanidins and Gut Microbiota

“…Thus, healthy individuals showed seasonally different responses in circulatory cytokines levels after endotoxin (lipopolysaccharides, LPS) administration, anti-inflammatory cytokine production being lower in summer compared to winter [ 14 ]. Moreover, a study in healthy humans showed that NK cells, interleukin 6, and peripheral blood monocular cells proliferation were increased under short days [ 15 ].…”